This invention relates to a process for activating a catalyst that is used in a hydrocarbon synthesis process starting from a mixture that comprises COxe2x80x94H3xe2x80x94(CO2), i.e., a mixture that comprises carbon monoxide and hydrogen and that optionally comprises carbon dioxide, called synthesis gas.
Such a synthesis generally makes it possible to obtain a mixture of saturated linear hydrocarbons, preferably essentially consisting of C5+ hydrocarbons (i.e., that have at least 5 carbon atoms per molecule).
It is known to one skilled in the art that the synthesis gas can be converted into hydrocarbons in the presence of catalysts that contain transition metals, preferably cobalt or iron. This conversion is known in the literature under the name of Fischer-Tropsch synthesis.
Many methods have been used in the past either for activating the new catalyst or for regenerating it after use. In most of the cases, this activation treatment, also called reduction, is carried out on the new oxidized catalyst and consists of a temperature treatment of the catalyst in the presence of pure hydrogen or in the presence of a gas that contains hydrogen.
Patents EP-A-0 168 894 and EP-A-0 152 652 thus show the possibility of improving the performance levels of the catalysts by using various partial pressure conditions of hydrogen and flow rates, conditions that can be variable during the entire reduction stage.
U.S. Pat. No. 5,168,091 describes the possibility of reducing the catalyst under hydrogen by keeping a partial pressure of water below 0.1 MPa.
Patents EP-A-0 533 227 and EP-A-0 533 228 describe a method for reduction under hydrogen that makes it possible to optimize the activity of the catalysts by adjusting the pressures, on the one hand, and the flow rates, on the other hand.
Patents WO 97/17137 and U.S. Pat. No. 5,389,690 describe the advantages of a slurry phase reduction, i.e., with a catalyst in suspension in a liquid phase that consists of hydrocarbons. The catalyst is reduced in a single stage, at a partial pressure of hydrogen that is greater than 1.5 MPa.
Finally, Patent WO 93/00993 describes the advantages of an activation in a single stage, in the presence of a gas that contains carbon monoxide and less than 30% of hydrogen, preferably in the presence of carbon monoxide alone. Before activation, the catalyst comes in oxidized form. The catalyst that is thus obtained has an improved activity and an improved C5+ selectivity.
This invention relates to a process for activating a catalyst that makes it possible to carry out the hydrocarbon synthesis starting from a mixture that comprises carbon monoxide, hydrogen and optionally carbon dioxide (Fischer-Tropsch synthesis). Such a synthesis generally makes it possible to obtain a saturated linear hydrocarbon mixture.
The activation process according to the invention comprises at least two stages:
at least one stage for activation in the presence of hydrogen, or a mixture of hydrogen and an inert gas, and
at least one stage for activation in the presence of carbon monoxide or a mixture of carbon monoxide and an inert gas,
and optionally a third activation stage that is carried out either in the presence of hydrogen or a mixture of hydrogen and an inert gas, or in the presence of carbon monoxide or a mixture of carbon monoxide and an inert gas.
This invention relates to a process for activating an active hydrocarbon synthesis catalyst starting from a mixture that comprises carbon monoxide and hydrogen, and optionally carbon dioxide. This synthesis is also called Fischer-Tropsch synthesis.
The activation process according to the invention comprises at least two stages:
at least one activation stage in the presence of hydrogen, or a mixture of hydrogen and inert gas, and
at least one activation stage in the presence of carbon monoxide or a mixture of carbon monoxide and an inert gas.
The order of these stages is unimportant, but preferably during the activation with the process according to the invention, the catalyst is not brought into contact simultaneously with hydrogen and carbon monoxide.
One of the preferred embodiments of the activation process according to the invention is described below.
According to this first preferred method, the first stage (stage 1) is carried out in the presence of hydrogen or in the presence of a mixture of hydrogen and inert gas, at a temperature of between about 10xc2x0 C. and about 700xc2x0 C., preferably between about 100xc2x0 C. and about 600xc2x0 C. and more preferably between 200xc2x0 C. and 500xc2x0 C., at a pressure of between about 0.05 MPa and about 30 MPa, preferably between about 0.1 and about 10 MPa, more preferably between 0.1 and 2 MPa, at an hourly volumetric flow rate of between about 20 and about 100,000 hxe2x88x921 (volume of mixture per volume of catalyst and per hour), preferably between about 100 and about 40,000 hxe2x88x921. The duration of the first stage is generally greater than 10 minutes, preferably between 1 and 24 hours, according to the selected conditions of flow rate and temperature.
According to this first preferred method, the second stage (stage 2) is carried out in the presence of carbon monoxide or in the presence of a mixture of carbon monoxide and an inert gas, at a temperature of between about 10xc2x0 C. and about 700xc2x0 C., preferably between about 100xc2x0 C. and about 600xc2x0 C., and more preferably between 180xc2x0 C. and 400xc2x0 C., at a pressure of between about 0.05 MPa and about 30 MPa, preferably between about 0.1 and about 10 MPa, more preferably between 0.1 and 2 MPa, at an hourly volumetric flow rate of between about 20 and about 100,000 hxe2x88x921 (volume of mixture per volume of catalyst and per hour), preferably between about 50 and about 40,000 hxe2x88x921. The duration of the first stage is generally greater than 10 minutes, preferably between 1 and 24 hours, according to the flow rate and temperature conditions.
Another preferred embodiment of the activation process according to the invention consists in carrying out a first stage in the presence of carbon monoxide or a mixture of carbon monoxide and an inert gas, and a second stage in the presence of hydrogen or a mixture of hydrogen and an inert gas.
The two stages of this second preferred method are carried out under the conditions that are described above. The first stage of this second preferred method is therefore carried out in the presence of carbon monoxide or a mixture of carbon monoxide and an inert gas, under the same conditions as stage 2 of the first preferred method. Likewise, the second stage in the presence of hydrogen or a mixture of hydrogen and an inert gas of this second preferred method is carried out under the same conditions as stage 1 of the first preferred method.
It is optionally possible to add a third stage to the activation process according to the invention. This third stage (stage 3) is also carried out either in the presence of hydrogen or a mixture of hydrogen and an inert gas, or in the presence of a carbon monoxide or a mixture of carbon monoxide and an inert gas, under conditions of temperature, pressure and flow rate that are identical to those that are indicated above for stages 1 or 2.
It is generally preferred to carry out at the end of each stage a purging under an inert gas to eliminate the traces of hydrogen or residual carbon monoxide.
All or part of these stages can be carried out in a gaseous phase or in a liquid phase. In the latter case, the catalyst is suspended in an inert solvent, for example a paraffinic fraction that preferably comprises at least one hydrocarbon that has at least 5, more preferably at least 10 carbon atoms per molecule, in particular and preferably in the case where a reaction for synthesis of hydrocarbons is used just after the activation stages and when this reaction is carried out in the presence of a liquid phase that preferably comprises at least one hydrocarbon that has at least 5, more preferably at least 10 carbon atoms per molecule.
The conversion of the synthesis gas into hydrocarbons generally takes place after the activation process and is generally operated under a total pressure that is usually between about 0.1 and about 15 MPa and preferably between about 1 and about 10 MPa, whereby the temperature is generally between about 150 and about 350xc2x0 C. and preferably between about 170 and about 300xc2x0 C.
The hourly volumetric flow rate is usually between about 50 and about 50,000 hxe2x88x921, preferably between about 100 and about 20,000 hxe2x88x921, and more preferably between 100 and 50,000 hxe2x88x921, and the molar ratio H2:CO in the synthesis gas is usually between about 1:2 and about 5:1, preferably between about 1.2:1 and about 2.5:1.
The activation process according to the invention that is described above can be used to activate a new catalyst in the oxidized state or to regenerate a used catalyst in the oxidized state or at least partly in the reduced state.
The catalyst is generally used either in the form of a calibrated fine powder that has a grain size of generally between about 10 and about 700 microns, or in the form of particles with an equivalent diameter that is generally between about 2 and about 10 mm. It is preferably used in the form of a calibrated fine powder when the activation process according to the invention is used in a liquid phase, and preferably in the form of particles when the activation process is used in a gaseous phase.
The catalyst generally comprises at least one metal of group VIII and a substrate. The element of group VIII of the periodic table is selected from among iron, cobalt and ruthenium. The metal of group VIII is preferably cobalt.
The substrate of the catalyst that can be used in the activation process according to the invention comprises at least one refractory oxide that is generally selected from among the oxides of magnesium, aluminum, silicon, titanium or zirconium, taken by themselves, mixed with one another or with oxides of other elements of the periodic table. The substrate that is used preferably will be selected from the group that consists of: alumina, silica, titanium oxide, carbon, aluminosilicates, and clays. Any other compound that can be used as a substrate can also be used, however. The substrate can be used in powder form or after shaping; any shaping technique that is known to one skilled in the art can be considered.
The catalyst that is used in the activation process according to the invention is preferably prepared by impregnation of at least one metal on a preformed substrate. A technique for preparing the catalyst is the impregnation of a solution that contains metal oxide particles and/or metal particles to be placed in suspension. The solvent can be an aqueous solvent, for example water, or an organic solvent.
The metal content of group VIII, expressed by weight of metal relative to the total weight of the catalyst, is generally between 0.1 and 50% by weight, preferably between 1 and 40% by weight, and more preferably between 5 and 30% by weight.
The catalyst can also contain other additional elements such as, for example, at least one alkaline metal, promoters such as, for example, at least one element that is selected from the group that consists of ruthenium, copper, molybdenum, tungsten, tantalum, titanium and scandium.
The content by weight of an additional element relative to the total weight of the catalyst is generally between 0.01 and 10% by weight, preferably between 0.1 and 5% by weight. These additional elements can be introduced at the same time as the metal of group VIII or in at least one subsequent stage.
In short, the process according to the invention is a process for activating a Fischer-Tropsch synthesis catalyst that comprises at least two stages:
at least one stage for activation in the presence of hydrogen, or a mixture of hydrogen and an inert gas, and
at least one stage for activation in the presence of carbon monoxide or a mixture of carbon monoxide and an inert gas.
In a first preferred embodiment, it can be a process for activating a Fischer-Tropsch synthesis catalyst that comprises at least two stages and in which:
the first stage (stage 1) is carried out in the presence of hydrogen or a mixture of hydrogen and an inert gas,
the second stage (stage 2) is carried out in the presence of carbon monoxide or a mixture of carbon monoxide and an inert gas.
In a second preferred embodiment, it can be a process for activating a Fischer-Tropsch synthesis catalyst that comprises at least two stages and in which:
the first stage (stage 1) is carried out in the presence of carbon monoxide or a mixture of carbon monoxide and an inert gas,
the second stage (stage 2) is carried out in the presence of hydrogen or a mixture of hydrogen and inert gas.
Stages 1 and 2 are preferably carried out at a temperature of between about 10xc2x0 C. and about 700xc2x0 C., at a pressure of between about 0.05 MPa and about 30 MPa, at an hourly volumetric flow rate of between about 20 and about 100,000 hxe2x88x921.
The activation process according to the invention can also comprise a third activation stage in the presence of hydrogen or a mixture of hydrogen and an inert gas. It can also optionally comprise a third activation stage in the presence of carbon monoxide or a mixture of carbon monoxide and an inert gas.
In a preferred way, a purging by an inert gas is carried out at the end of each stage of the process according to the invention.
According to one of the embodiments of the process according to the invention, a portion, and even all, of the stages can be carried out in gaseous phase. According to another embodiment of the process according to the invention, a portion and even all of the stages can be carried out in liquid phase. In the latter case, the liquid phase is preferably a paraffinic fraction. More preferably, said liquid phase comprises hydrocarbons that have at least 5 carbon atoms per molecule and more preferably at least 10 carbon atoms per molecule.
The process according to the invention makes it possible to activate any Fischer-Tropsch synthesis catalyst. It is particularly well suited, for example, for activating a catalyst that comprises at least one metal of group VIII and a substrate.